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Figure 1. A dog with severe weight loss caused by small intestinal disease.

Inflammatory bowel disease (IBD) has been an important topic of discussion and research in canine and feline gastroenterology for more than 20 years. At times, IBD has been stated to be the most common cause of chronic gastrointestinal signs in dogs and cats. Small intestinal IBD can be particularly devastating, as severe weight loss with or without hypoalbuminemia may occur (Figure 1).

One of the problems with IBD is that different definitions have been applied, and there has been no clear consensus as to exactly what constitutes IBD. In some cases, IBD has been referred to in the context of a histologic diagnosis, diagnosed solely by finding inflammation in intestinal biopsies (Figures 2 & 3); while in other cases, IBD has been defined as idiopathic intestinal inflammation, in which histology by itself is inadequate for diagnosis. Our current collective term for canine and feline IBD describes persistent or recurrent gastrointestinal signs that have histologic evidence of inflammatory infiltration of the small or large intestinal mucosa of unknown cause. Current thought is that the condition can only be called IBD if no underlying cause for the inflammation can be found.1

The role of gut bacteria

Figure 3. The endoscopic appearance of the duodenal mucosa in a dog with severe mucosal inflammation. In this case, no cause could be found, and the diagnosis became IBD.

Recently, new evidence suggests that intestinal bacteria probably play an important role in the initiation or perpetuation of the intestinal inflammation that has been termed IBD.1-5 Unfortunately, histology by itself cannot distinguish idiopathic intestinal inflammation from intestinal inflammation due to these gut bacteria or diet.

With the advantage of hindsight, we can now see that the effectiveness of metronidazole in treating IBD should have been an early clue to the potential importance of bacteria as a cause of IBD. Metronidazole has benefitted many patients diagnosed as having IBD because of what was hypothesized to be an immunomodulatory effect.6 However, current thought is that metronidazole may be beneficial in these patients because of its efficacy in killing anaerobic bacteria. Another clue has been the finding that many dogs and some cats with steroid-resistant IBD may respond better to an elimination diet trial or antimicrobial therapy than to anti-inflammatory drugs.

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While much of this evidence is anecdotal, there are published accounts of antibiotics (e.g. tylosin) "curing" what was diagnosed as IBD dating back 30 years.7 Since then, the efficacy of tylosin has been confirmed in other studies,8 which has helped reveal the potential role that bacteria play in causing or maintaining the intestinal inflammation in patients with IBD.1-5 Recently, the term dysbiosis has been suggested as a better description of what is occurring in the intestines of patients with IBD.

Dysbiosis refers to an imbalance in the commensal intestinal bacterial population such that bacterial species that tend to favor inflammation in the intestinal mucosa (e.g. Escherichia coli) are overrepresented while those species that are less likely to induce inflammation (e.g. Bifidobacterium species) are underrepresented. The result is initiation and/or perpetuation of intestinal inflammation.